Hello!
at first. i`m from Austria/europe.
i bought my 75 coupe deville powered with a 500cui engine about one and a half year ago.
but now i want some more power! i think about 500 to 600 hp.
please tell me what i need to replace and where i can get it.
it should be cheep but still working for long time.
and do i have to replace the transmission?
please help me.

Go to www.cad500parts.com all your questions will be answered in the Dyno section of the on line catalog.
The TH-400 is a tough tranny. Keep it cool with a big cooler and it should work for awhile.

There is a Potter Automotive who sells a cadillac 500 crate motor with 500+ HP... This can give you an idea what needs to be done... and there also is MTS who also sells many aftermarket parts for these motors...I suggest you do a lot of searching and questions before you buy anything..